based from this site http://www.memoryx.net/asusipibllb.html
it seems that the supported and possible processor upgrades are
- Intel Core 2 Quad Q9xxx Processor
- Intel Core 2 Duo E8xxx Processor
- Intel Core 2 Quad Q6xx0 Processor
- Intel Core 2 Duo E6x00 Processor
- Intel Core 2 Duo E4x00 Processor
unfortunately, there isnt much official information on the Asus or HP website. that motherboard is supposedly created by Asus specially for HP -- hence there isnt much official specifications with regards to the supported hardware.
you may want to choose any of those processor ranges, but do take note, that the newest processors today are the i3-i5-i7 family of intel processors. these newer family of intel processors also run on newer motherboards.
however, there is not particular requirement yet that requires the new i series of processors. any of the possible upgrades are still good for the usual games and casual photo/video editing.
if you are looking into playing games with that machine, you may want to check the available video card upgrades. http://whatgfx.com/
is a good starting point for selecting a video card upgrade. remember tho, that its probably best to pick a game that you will most likely be playing on that machine. if the game requirements meet your current processor, then a video card would be the better upgrade choice.
once you have the video card as required by your game, you can check the other model cards within its price and performance range. compare them with http://www.gpureview.com/
of course you can also maximize the amount of RAM -- which would of course need you to upgrade to a 64bit OS to take advantage of more than 3GB ram.
goodluck on this